Cold Front and Downbursts in Oklahoma

During the afternoon of 30 June 2009, strong convective storms developed along a weak cold front as it was tracking southward over Oklahoma. Although there was very little temperature contrast across the front, the front acted as a convergence zone and a trigger for deep, moist convection. The pre-convective environment downstream of the cold front over western Oklahoma was dominated by vertical mixing that fostered the development and evolution of a convective boundary layer. Elevated Geostationary Operational Environmental Satellite (GOES) imager brightness temperature difference (BTD) values and Microburst Windspeed Potential Index (MWPI) values in the vicinity of downburst occurrence over western Oklahoma served as evidence of the presence of a well-developed mixed layer. Strong downbursts that were recorded by Oklahoma Mesonet stations between 2100 UTC and 0000 UTC July 1 resulted from sub-cloud evaporation of precipitation. These downbursts occurred in proximity to moderate to high microburst risk values as indicated in the 2000 UTC GOES microburst products.


The images above are a Geostationary Operational Environmental Satellite (GOES) imager microburst product (top) and a corresponding GOES sounder Microburst Windspeed Potential Index (MWPI) product at 2000 UTC 30 June 2009 (bottom), with the location of Oklahoma mesonet observations (WEAT, CHEY, MANG) of downburst wind gusts plotted on the images. The GOES imager microburst product displayed convective clouds developing along a weak cold front over western Oklahoma that would soon trigger strong convective storms. Downburst wind gusts between 39 and 44 knots were recorded by the Oklahoma Mesonet stations plotted in the images above between 2100 UTC and 0000 UTC July 1.

The Oklahoma Mesonet meteogram above displays surface characteristics of the strongest downburst that occurred at Mangum at 2350 UTC. Classic downburst signatures are apparent in the meteogram including a sharp peak in wind speed (51 mph/44 knots), significant temperature decrease (27F), and a pressure jump of 4 millibars (mb).

The 2200 UTC GOES imager microburst product with overlying radar reflectivity imagery from Frederick, OK (KFDR) NEXRAD above displays a small, well-defined bow echo associated with downburst occurrence at Mangum. Note that this downburst occurred in a region of elevated output BTD (near 40K, orange shading), signifying the presence of a mature convective mixed layer (ML) as illustrated in the sounding profile below.


Elevated risk values indicated the presence of a dry subcloud layer with a steep temperature lapse rate up to 600 mb that favored the development of intense downdrafts due to the evaporation of precipitation and resulting negative buoyancy. This favorable environment was most effectively shown in the above sounding profile over Mangum at 2200 UTC.